The nasal cavity of most mammals contains more than one chemoreceptor system. The olfactory system is complemented by the vomeronasal system, and by a third putative chemoreceptor system the nervus terminalis. The olfactory and vomeronasal systems, at least, are involved in chemical communication between individuals of the same species (pheromone communication). Communication by chemical signals in humans is often dismissed on introspective evidence but subliminal effects may occur as in menstrual synchrony in women who live together. Recent reports suggest that contrary to the conventional wisdom of the last several decades, there is a vomeronasal organ in most if not all human adults and there is some indication of response to chemicals, although its function as a "pheromone detector" is as yet unproven. Whether or not the human vomeronasal organ is eventually shown convincingly to be functional, the system is of great importance in animals and furthermore, the parts of the mammalian brain that evolved to receive vomeronasal input, the corticomedial amygdala, still has a role to play in the human brain. The nervus terminalis, which in mammals is intimately associated with the vomeronasal system, is also present in humans, where it constitutes the only undisputed cranial nerve of unknown function. Basic information on the original function of these systems, which evolved in earlier vertebrates may help to elucidate their present function in humans - in addition to the knowledge gained on basic neurobiological mechanisms. Male hamster reproductive behavior is the model system for these studies. Experiments proposed here include: (1) The investigation of chemosensory system contributions to patterns of neuronal activation in central chemosensory pathways in the brain. Active neurons in animals with and without functional vomeronasal, olfactory or terminalis chemoreceptor systems, will be marked by the expression of the rapidly induced gene, c- fos, in response to natural stimuli to their components and to electrical stimulation of chemoreceptor input; (2) A study of the influence on these patterns of LHRH, a hormone which facilitates reproductive behavior and appears to be released into the brain by vomeronasal (and possibly nervus terminalis) activation; (3) Exploration of the intracerebral functions of LHRH, a) by identifying LHRH cells activated by chemosensory nerve input, using immunocytochemistry and in-situ hybridization; and b) by eliminating LHRH target cells within the brain using conjugated LHRH to selectively deliver the cytotoxin Ricin-A-chain to cells with LHRH receptors. The behavioral effects and the effect on c-fos expression patterns will be studied together with the location and projections of the LHRH-target cells; (4) Continue the study of the effect of intracerebrally injected LHRH on reproductive behavior, including the identification of its site of action and its interaction with other systems controlling prolactin; (5) Direct investigation of the ability of vomeronasal (and/or Nervus terminalis) activation (electrical stimulation) to alter the release of LHRH and other substances in the brain as measured by radioimmunoassay of LH and prolactin in the blood, released via hypothalamic influence on the pituitary gland.